An electronic apparatus may include a DC-DC converter (switching regulator) that generates a voltage which is higher or lower than an output voltage of a battery. The output voltage of the battery varies depending on a left amount of stored power.
An external power source such as an AC adaptor outputs a fixed voltage. The DC-DC converter generates a fixed output voltage even if an input voltage varies.
A method for controlling a switching element of the DC-DC converter includes a Pulse Width Modulation (PWM) method and a Pulse Frequency Modulation (PFM) method. According to the PWM method, the output voltage of the DC-DC converter is compared to a reference voltage as a target value, and a pulse width of a drive signal varies in such a way that an error voltage is minimized. When a switching frequency is set to be fixed and a duty ratio of ON-time of the switching element varies, a voltage rise rate varies depending on the input voltage so that the output voltage is maintained fixed.
According to the PFM method, to obtain excellent conversion efficiency in a low load state in which a load current is reduced, the switching frequency varies depending on the stage of the load. In the low load state, switching operation of the switching element stops for a predetermined period, and a consumed power (consumed current) is reduced.
To avoid noises, the switching frequency may be excluded from a human audible frequency band, for example, several tens of Hz to 20 kHz.
The related technique is disclosed in Japanese Laid-open Patent Publication No. 2005-168129 and the like.